Biotypes of Amaranthus retroflexus L ., A. hybridus L., A. bouchonii Thell. and Chenopodium album L. insensitive to atrazine were collected from maize monoculture where atrazine had been applied extensively. Atrazine-resistant biotypes of A. retroflexus and A. hybridus showed phenmedipham and lenacil co-resistance and atrazine-resistant biotype of C. album showed fenuron co-resistance. An atrazin-resistant biotype of A. bouchonii with co-resistance to diuron was not resistant to fenuron, lenacil and phenmedipham.